Unsafe foreign calls (fat machine instructions) do not kill all registers.
The new code generator was doing some interesting spilling across unsafe foreign calls: _c1ao::I32 = Hp - 4; I32[Sp - 20] = _c1ao::I32; foreign "ccall" newCAF((BaseReg, PtrHint), (R1, PtrHint))[_unsafe_call_]; _c1ao::I32 = I32[Sp - 20]; This is fairly unnecessary, and resulted from over-conservative liveness analysis from CmmLive. We can see that the old code generator only saved volatile registers across unsafe foreign calls: spilling variables was done by saveVolatileVarsAndRegs, which was only performed for ordinary calls. This commit removes the excess kill from the liveness analysis, as well as the *redundant* excess kill from spilling-and-reloading, and adds a note to CmmNode to this effect. The only registers we need to kill are the ones that the foreign call assigns to, just like any other machine instruction. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>